The One Cycle Control (OCC) technique for controlling switching circuits is now known. The general technique is described in U.S. Pat. No. 5,278,490. The technique is applied to a PFC (Power Factor Correction) boost converter in U.S. Pat. No. 5,886,586. In the OCC technique, as applied to a PFC boost converter circuit, the output voltage of the converter is sensed, compared to a reference voltage and supplied to an integrator stage, which is reset for each control cycle as set by a system clock. The integrator output is then compared in a comparator to the sensed input current in the converter and the output of the comparator is provided to control a pulse width modulator, whose output controls the boost converter switch. The switch controls the current supplied to the load so that the input ac line current is in phase with the input ac line voltage, i.e., the converter with attached load has a power factor of substantially unity and thus appears purely resistive, resulting in optimum power efficiency as well as reduced harmonics.
Prior to the OCC technique, a multiplier technique was known for PFC in a boost converter circuit. FIG. 1 depicts a system level block diagram representation of a typical prior art active power factor correction system operating in a fixed frequency, continuous conduction mode (CCM) boost converter topology. The system consists of a continuous conduction mode control integrated circuit 1, based on a multiplier approach, with discrete gate drive circuitry 3 and discrete power switch 5. This control method, based on current mode control, employs the use of a multiplier circuit, input current sensing, input voltage sensing, and output voltage sensing. The analog multiplier creates a current programming signal by multiplying the rectified line voltage by the output of a voltage error amplifier such that the current programming signal has the shape of the input voltage, and average amplitude, which ultimately controls the output voltage of the boost converter. The current loop is programmed by the rectified line voltage such that the input of the converter appears resistive. The output voltage is controlled by changing the average amplitude of the current programming signal. The result is a regulated output voltage and sinusoidal input current in phase and proportional to the input voltage.
The above prior art using the multiplier approach has the disadvantage of high discrete component count and complicated design and development effort required to implement high performance continuous conduction mode power factor correction converters. In addition, it is more difficult to implement a “single package” design wherein the switch is packaged with the control circuit because of the high component count and numbered pins. An example of a prior art PFC boost converter using the multiplier technique is shown in U.S. Pat. No. 6,445,600.
In U.S. patent application Ser. No. 10/319,982 filed Dec. 16, 2002, assigned to the assignee hereof, a PFC boost converter circuit employing the OCC technique is described comprising an integrated circuit for a PFC, CCM boost converter having an integrated switch and OCC controller and which reduces the complexity inherent in a PFC boost converter employing the multiplier technique.
There is a need, however, to incorporate additional features into this integrated circuit, including circuits to control inrush current, fan motor speed and thehousekeeping power supply that powers internal circuits. It has been realized that the simplicity of the OCC control method allows integration of these additional “peripheral” functions often implemented external to the PFC Controller IC. This increases the value of such a controller compared to conventional multiplier based controllers given that there is more functionality per unit area of silicon.
The OCC technique dramatically simplifies the continuous conduction mode (CCM) PFC control function compared to conventional “multiplier” based CCM PFC controllers such as the prior art Unitrode/TI UC3854. The number of package pins are reduced dramatically since the OCC technique does not require line voltage sensing and does not need a complicated multiplier circuit with all its associated external components. The simplicity of this control method thus allows a higher level of integration while allowing use of practical pre-existing power packaging methods for full integration of the CCM boost PFC controller along with the additional peripheral functions referred to above. In addition, the power switching element may also be integrated in the package. An example of a packaging method that can be employed is shown in International Publication WO 01/39266 published May 31, 2001.